Also see section 5.5.2 (Absence of Router Advertisements) in RFC 4862. Basically, the assumption is that the dhcp client still needs to learn the gateway through RA, so you still need to enable RA on the router.

If you can't ping the gateway, then it's unlikely an RA or DHCPv6 problem. If you statically configure the workstation with an IPv6 address on the same network as the gateway, does it work? If not, can you take a packet capture to see if you get the NS/NA packets as you are supposed to?

I think you should configure a link-local IPv6 address on the router interface. Then the client can learn the default gateway through router's RA and use the router's link-local address as the default gateway. You cannot get the default gateway info through DHCP v6.

Do you mean this PC can't communicate with other workstations in other subnets?

If that is the case, yes you need to configure link-local address on other connected interfaces on the router. In this way, all workstations in each subnet can get the default gateway(router's link-local address) and use that to communicate with any workstations in different subnets.